The present invention relates to an arc tube for a discharge lamp device.
FIG. 8 shows a conventional discharge lamp device. The discharge lamp device has a structure in which front and rear end portions of an arc tube 5 are supported by a pair of lead supports 3 and 4 projecting forward from an electrically insulating base 2. The arc tube 5 has a structure in which a closed glass bulb 5a is formed between a pair of pinch seal portions 5b, 5b such that a pair of electrode rods 6, 6 are disposed so as to be opposite to each other in tire glass bulb 5a by the pinch seal portions 5b, 5b respectively and luminous materials are enclosed in the glass bulb 5a. A piece of molybdenum foil 7 integrally connected to the electrode rod 6 and a lead wire 8 is enclosed in each of the pinch seal portions 5b. A circular arc generated between the electrode rods 6, 6 in the closed glass bulb 5a emits light to thereby provide a turned-on state.
A method for producing the arc tube 5 is disclosed, for example, in Japanese Patent Application Laid-open No. Hei. 6-231729. As shown in FIG. 9(a), first, an electrode assembly A constituted by an electrode rod 6, a piece of molybdenum foil 7 and a lead wire 8 to which the rod 6 and the foil 7 are integrally connected is inserted into a cylindrical glass tube W from one opening end side of the glass tube W. The glass tube W has a spherically swollen portion w.sub.2 formed in the middle of the glass tube W, that is, between linear extension portions w.sub.1. A position P.sub.1 near the spherically swollen portion w.sub.2 is primarily pinch-sealed. Then, as shown in FIG. 9(b), luminous materials P, etc., introduced into the spherically swollen portion w.sub.2 from the other opening end side of the glass tube W. Then, as shown in FIG. 9(c), another electrode assembly A is inserted and, at the same time, a position P.sub.2 near the spherically swollen portion w.sub.2 is heated and secondarily pinch-sealed while the spherically swollen portion w.sub.2 is cooled by liquid nitrogen so that the luminous materials are not vaporized. In this manner, the spherically swollen portion w.sub.2 is sealed hermetically, so that an arc tube 5 having a tipless closed glass bulb 5a is finished.
Another method for producing the arc tube is disclosed, for example, in Japanese Patent Application Laid-open No. Hei. 5-174785. As shown in FIG. 10(a), first, an exhaust tube w.sub.3 is integrally connected to a spherically swollen portion w.sub.2 of a cylindrical glass tube W to produce a T-shaped glass tube. Then, as shown in FIG. 10(b), a pair of electrode assemblies A are inserted from the respective opening end sides of linear extension portions w.sub.1. As shown in FIG. 10(c), portions of the respective linear extension portions w.sub.1 near the spherically swollen portion w.sub.2 are pinch-sealed (as indicated by arrows). Incidentally, FIG. 10(c) is a bottom view of the T-shaped glass tube shown in FIG. 10(b). Then, as shown in FIG. 10(d), luminous materials P, etc., are introduced through the exhaust tube w.sub.3. Then, as shown in FIG. 10(e), the exhaust tube w.sub.3 is tipped off, so that an arc tube having a tipped and closed glass bulb is finished.
In the aforementioned conventional arc tubes, however, luminous flux, light color, etc., vary largely in accordance with the produced arc tubes and the proportion defections are increased correspondingly.
The present inventor has examined the cause of the high percentage of defections. As a result, it has been found that the cause is in that, in the step of pinch-sealing the linear extension portions w.sub.1 of the glass tube W, a glass material in the pinched linear extension portion w.sub.1 side flows into the spherically swollen portion w.sub.2 side to thereby make the internal shape of the closed glass bulb 5a distorted or make the electrodes (electrode rods 6, 6) eccentric from each other with an amount of eccentricity L as shown in an enlarged view in FIG. 11.
That is, as shown in FIGS. 12(a) and 12(b), a heated region of a linear glass tube is shaped up spherically by blow molding to thereby obtain the cylindrical glass tube W used in the production of an arc tube. Accordingly, neck portions 9 as shown in enlargement in FIG. 13 are formed in the boundaries between the linear extension portions w.sub.1 and the spherically swollen portion w.sub.2. The inner diameter d.sub.9 of the neck portions 9 is, however, formed to be several times as large as the diameter of spherical portions 6a at the ends of the electrode rods 6 so that the spherical portions 6a can be inserted into the spherically swollen portion w.sub.2 smoothly. Accordingly, the clearance between the electrode rods 6 and the neck portions 9 is so large that when the glass material is pinched in the pinch-sealing step, the heated and softened glass material in the linear extension portion side flows into the spherically swollen portion w.sub.2 side (see the arrow in FIG. 11) to make the internal shape of the closed glass bulb 5a distorted or make the counter electrodes eccentric from each other.
Particularly, in order to produce an arc tube 10 having a tipless closed glass bulb 12, the glass tube W must be secondarily pinch-sealed while the spherically swollen portion w.sub.2 is cooled so that the introduced luminous materials, etc., are not vaporized. Accordingly, the inner pressure of the spherically swollen portion w.sub.2 becomes negative at the time of pinch-sealing. As a result, the pinched glass material is apt to be sucked into the spherically swollen portion w.sub.2 correspondingly. The problem that the internal shape of the closed glass bulb 5a becomes distorted or the counter electrodes become eccentric arises remarkably in the arc tube 10 having such a tipless closed glass bulb.
Therefore, the present inventor conducted experiments and made considerations upon the aforementioned problems in the prior art. As a result, the present inventor confirmed that the aforementioned problems could be solved if the inner diameter of the neck portions 9 between the linear extension portions w.sub.1 and the spherically swollen portion w.sub.2 in the glass tube W was reduced. Thus, the present inventor has achieved the present invention.